Maybe we are getting ahead of ourselves. I think you need to isolate the components one by one. Ace suggested the rockers, but they seem ok. So the next in the path would be the push rods, cam gears. But since you have found that there is a difference in movement dependent upon which gear it is in, that leads me to believe it could be in the gearbox. To isolate the gearbox from the cam/pushrod/ crank assembly, you could remove the clutch/primary chain. That job would take about 2hrs if you haven't done it before.
If still no movement with primary chain removed, that would direct you to the timing chest, I think.
If , on the other hand, you get movement of the piston with primary chain removed, that would direct you to the gearbox (broken tooth perhaps). Getting inside the gearbox would take about one hour.
So, I think your dilemma is to either:
1) remove the primary chain to determine if the problem is in the gearbox or in cam/pushrod/ crank assembly .
2)Take a chance and open the gearbox looking for broken teeth.
Now that is just my opinion, and remember, I am no mechanic. So, it is up to you to roll the dice. What do others in the group think the next step should be?